1. Field of the Invention
The present invention relates to etching methods used in the fabrication of integrated electronic circuits on a semiconductor substrate such as silicon, particularly a single-chamber/single-cathode (in situ) method of anisotropically plasma etching a sandwich structure of an oxide, tungsten silicide, and polycrystalline silicon, or equivalent structure.
An electronic circuit is chemically and physically integrated into a substrate such as a silicon wafer by patterning regions in the substrate, and by patterning layers on the substrate. These regions and layers can be conductive, for conductor and resistor fabrication, or insulative, for insulator and capacitor fabrication. They can also be of differing conductivity types, which is essential for transistor and diode fabrication. Degrees of resistance, capacitance, and conductivity are controllable, as are the physical dimensions and locations of the patterned regions and layers, making circuit integration possible. Fabrication can be quite complex and time consuming, and therefore expensive. It is thus a continuing quest of those in the semiconductor fabrication business to reduce fabrication times and costs of such devices in order to increase profits. Any simplified processing step or combination of processes at a single step becomes a competitive advantage.
2. State of the Art
A situation where a process simplification is desirable is in the anisotropic etch of a layer of oxide on a layer of silicide on a layer of poly (also called an oxide/silicide/poly sandwich structure). In this disclosure, xe2x80x9coxidexe2x80x9d denotes an oxide of silicon, xe2x80x9csilicidexe2x80x9d is short for tungsten silicide, and other commonly known silicides such as tantalum silicide, molybdenum silicide, and titanium silicide, and xe2x80x9cpolyxe2x80x9d is shoptalk for polycrystalline silicon. xe2x80x9cPolycidexe2x80x9d denotes a silicide-over-poly combination. Oxide is an insulator with dielectric properties. Poly is resistive in nature, but is made less resistive when doped with an element having less or more than four valence electrons, or when layered with conductive silicide.
An oxide/silicide/poly sandwich structure presents a difficult etching task, particularly with an additional mask layer of photoresist (xe2x80x9cresistxe2x80x9d), which must be the case if patterning is desired. The difficulty is due to the distinct differences in the way oxide and polycide are etched, particularly with resist still present on top of the structure.
Both oxide and polycide can be etched using a parallel plate plasma reactor. However, an oxide is typically etched in fluorine-deficient fluorocarbon-based plasmas, whereas silicide and poly can be etched in fluorine- or chlorine-based discharges. Reactor cathode materials may also differ: for oxide etch, an erodible cathode such as graphite or silicon is often used to provide a source of carbon or silicon for etch selectivity, whereas for polycide etch, an inert cathode is preferred, especially when utilizing chlorine gas (Cl2) for selectivity. If a single-chamber process were attempted using conventional art to etch an oxide/silicide/poly sandwich structure, the erodible cathode required for oxide etch would be destroyed by the chlorine required for polycide etch. Using conventional methods, the two steps are incompatible.
Oxide etch in general is fairly well understood given a universal need for a vertical profile. This vertical profile is realized primarily by ion induced reaction with the oxide, coupled with normal incidence of the ions on the oxide surface. The amount and energy of these ions are primarily controlled by the reactor""s rf power and gap. Generally, a fluorocarbon-based gas mixture is introduced at a low pressure into the etch chamber. The exact gas composition is chosen, and an erodible cathode is used to scavenge excessive fluorine radicals so that the fluorine-to-carbon ratio is near, but not beyond, the so-called polymerization point. Under these conditions, when a plasma is ignited, the fluorocarbons are dissociated and release fluorine radicals and CFx species. Although fluorine radicals etch oxide, they do so very slowly: the primary etchant for oxide is considered to be the CFx species. Some of these species diffuse to the oxide surface where, with the assistance of ion bombardment, they react with the oxide and release volatile byproducts SiF4, CO, and CO2. In addition, some of the CFx species react with each other to form fluorocarbon polymers. Polymer that forms on horizontal surfaces is removed by vertical ion bombardment. Polymer that forms on vertical sidewalls is not significantly degraded by the bombardment, and actually serves a useful purpose by protecting the sidewalls from attack by the etchant species. This sidewall protection enables the achievement of vertical profiles, adjustable by varying the fluorine-to-carbon ratio. As the cathode is eroded, the quantity of available fluorine radicals is reduced. Therefore, a polymer-producing gas such as CHF3 is balanced against a fluorine-producing gas such as CF4 to provide proper selectivity, with assistance to sidewall protection.
Two methods are presently used to etch an oxide/silicide/poly sandwich structure. Both methods use separate reactors: one for oxide etch and one for polycide etch. The first method involves etching the oxide in an oxide etch reactor, using an erodible cathode. After oxide etch, the resist is removed from the wafer. Silicide and poly are then etched in a poly etch reactor, using an inert cathode. Both etches are anisotropic.
The second method uses the same principles as the first, except that there are two reactors in one machine. The two reactors are configured as separate oxide and polycide reactors having a common vacuum transfer area, so that a wafer can be transferred in a vacuum from the oxide reactor to the polycide reactor, thus minimizing additional handling. The resist is generally not removed prior to polycide etch in this method. This is sometimes referred to as xe2x80x9cin situxe2x80x9d since the wafers never leave the vacuum of one machine. However, they are etched in two different etch chambers and are not truly in situ in the sense of this disclosure.
It would be of great advantage to etch oxide and polycide truly xe2x80x9cin situ,xe2x80x9d that is, in one reactor chamber, with a single cathode.
An object of the present invention is to provide a method of anisotropically etching an oxide/silicide/poly sandwich structure in situ. Other objects of the invention are a fast processing time, an improved process yield and cleanliness.
In summary, the inventive process is as follows. A wafer having the sandwich structure described above, coated with a mask layer of resist, is transferred into the chamber of a parallel plate plasma reactor, having an anodized aluminum cathode and a variable gap, for two steps: oxide etch and polycide etch. In the oxide etch step, oxide not protected by resist is exposed to a plasma of about 1.9 W/cm2 power density at a 0.48 cm gap, in a 2.3 torr atmosphere of 50 sccm C2F6, 100 sccm He, 40 sccm CF4, and 32 sccm CHF3. Immediately after the oxide etch step, in the same chamber and using the same cathode, silicide and poly layers are etched in a plasma of about 0.57 W/cm2 at a 1.0 cm gap in a 0.325 torr atmosphere of 90 sccm Cl2 and 70 sccm He. The finished structure has a vertical profile at or near 90xc2x0 from horizontal, with no bowing or notching. The entire inventive process takes about 3 minutes.